The present invention relates to an apparatus for compression molding recorded discs and particularly to such an apparatus which has center hole forming means with improved wear.
A high information density recorded disc, such as a video disc, has a packing density of the information track in the order of 10,000 rotations per inch (3,937 per cm) so that the information track must be much finer in dimensions than that of an audio disc, and rotates at a higher speed, about 450 rpm during playback, as compared to 33 rpm for an audio disc. Because of the fine dimensions of the information track and the high speed rotation of the disc, the disc must be stably supported to insure proper tracking of the stylus during playback. To help achieve this stability, the disc is provided with a relatively large center hole, about 1.3 inches (3.3 cm) in diameter and is seated in the player on a truncated conical spindle.
A method generally used to make the disc is compression molding. It has been found desirable to form the center hole in the disc as it is being molded. An apparatus suitable for forming the center hole in the disc as it is being molded is shown and described in copending applications for U.S. Patent of Michael Lee McNeely, Ser. No. 93,012, filed Nov. 9, 1979, METHOD AND APPARATUS FOR PRODUCING DISC RECORDS HAVING MOLDED-IN CENTER HOLES, and Robert Winfield Chambers et al, Ser. No. 187,161, filed Sept. 15, 1980, APPARATUS FOR MOLDING A RECORDED DISC, which are incorporated herein by reference.
The apparatus of each of the above applications includes a pair of mold plates, a center hole forming pin slidably mounted in one of the mold plates and a center plate in the other mold plate. The center hole forming pin and the center plate have surfaces which mate when forming the hole in the disc. A problem with this apparatus is that the mating surfaces of the center hole forming pin and the center plate which form the hole in the disc become worn during repeated operation of the apparatus. This causes an undesirable variation in the size of the hole formed in the disc and as well affects the surface quality of the hole formed.
Thus, when a center hole forming pin and/or a center plate becomes worn, it must be replaced which is costly with respect both to parts and down time of the apparatus. Also, the center hole forming pin is made up of an end portion which is threaded on the end of a pipe with the passage in the pipe being aligned with a passage in the end portion so that steam and water can flow to the end portion to heat and cool it. It has been found that during the operation of the apparatus there has been a tendency for the center hole forming pin to develop a leak across the junction between the end portion and the pipe. This can cause steam or water to drip onto the disc being molded, which can adversely affect the disc.